He has a full videos with step by step instructions on building one with a GQF wafer thermostat and one with a water heater thermostat.

I have it running right now with 7 eggs in it and it works great: I only get 1° variance.
The one piece of advise I recomend is make sure you get a [ulower][/u] water heater thermostat. I started out with an upper and the temp variance was over 6°.

The wafer thermostats are more reliable. I used to use a lower wter heater thermostat but since I got the wafer I'm glad I got it.Wit the wafer you won't have to drill any excess holes in it to make it more acurate.Just set it & forget it. You see the wafer wired between the lights.